Humidity (Update)/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby. Tim and Moby sit in a car on a snowy day. Their car is one of many on a boat sailing to the Yukon. TIM: Oh boy, I’m so excited about this trip! I’ve always wanted to explore the Yukon. Tim and Moby exit the car. Tim shivers in the cold. TIM: On second thought, maybe northern Canada isn't the best place to visit for winter break. Moby hands Tim a letter, and Tim reads from it. TIM: Dear Tim and Moby, Why is humid heat more uncomfortable than dry heat? From, Parker. I don’t know, Parker, some humid heat sounds pretty comfy to me right now! MOBY: Beep? Moby extends a coat and scarf to Tim. Tim puts them on and stops shivering. TIM: Humidity is the amount of moisture in the air. It’s part of the water cycle–the movement of water around the Earth. Water is constantly on the go. An animation shows a large body of water. Small water molecules lift into the air and form a cloud. TIM: It moves around in space: between the oceans, land, and air. Wind blows the cloud. It starts to rain. TIM: And also between different states: liquid, solid, and gas. Icons for the three states of matter appear. The molecules in the gas icon are spaced out and move quickly. The molecules in the liquid icon are close together and move less quickly. The molecules in the solid icon are tightly packed and only vibrate slightly. TIM: Humidity is one stage of the cycle: it’s the water that’s in the air in gas form, also known as water vapor. An animation shows gas molecules in the air. The molecules are spread out and move quickly. TIM: Some places have very little water, like deserts. So the humidity is low, and the air is dry. An animation shows a desert. An inset bubble demonstrates that very few vapor molecules are in the air. TIM: In other places, like rainforests, there's lots of water. So the air tends to stay really humid. An animation shows a rainforest. Its inset bubble demonstrates that many vapor molecules are in the air. MOBY: Beep? TIM: Zoom in real close and we’ll see it at work. Water, like everything else, is made of individual particles, called molecules. The molecules of liquid water are all stuck together, but kinda jiggling around. An animation magnifies a water droplet on Tim's hand. Hundreds of tightly packed molecules wiggle around. TIM: Some of these molecules move faster than others. When one of them breaks free and escapes into the air, it becomes vapor. This is called evaporation: liquid water turning into gas. An animation shows a fast water molecule evaporating. TIM: Since gas molecules are spread out and disconnected, they move a lot faster than those in a liquid. In other words, they have more energy. An animation shows vapor molecules in the air. They move more quickly than the water molecules. TIM: But just as in liquids, some vapor molecules move faster, and some go slower. The slowest ones tend to get stuck¬¬¬–to each other, or to bodies of liquid water. A few vapor molecules join together. TIM: Once two or more of them link up, they turn to liquid, in a process called condensation. An animation shows condensed water molecules as dew drops on a plant. TIM: Evaporation and condensation are constant processes that drive the Earth's water cycle. An animation shows a pond. An arrow pointing up symbolizes evaporation from the pond. An arrow pointing down symbolizes condensation from the air. TIM: But in any given time and place, there's usually more movement in one direction than the other. Either more water is evaporating, making the air more humid… The evaporation arrow becomes thicker than the condensation arrow. More evaporation occurs than condensation. TIM: Or more vapor is condensing, making the air drier. The condensation arrow becomes thicker than the evaporation arrow. More condensation occurs than evaporation. MOBY: Beep? TIM: It's not exactly random. The system is trying to reach equilibrium, or balance. That's what happens if we seal it up in a closed container. The animation reveals that the river scene is inside a jar. TIM: The number of molecules going up and going down will gradually even out. And it'll stay that way forever, as long as conditions remain unchanged. The rates of evaporation and condensation change until the pond system reaches equilibrium. TIM: But our environment is not a sealed container. Changes in temperature, air pressure, and wind keep water cycling around. An animation shows another pond scene. The water level rises and falls because the balance of evaporation and condensation changes. TIM: On Earth's surface, the balance is almost always tipped toward evaporation. The evaporation arrow thickens as the water level drops. More evaporation is happening than condensation. TIM: That's why if you leave a glass of water out, it'll eventually disappear. The only question is how fast it'll happen. An animation shows a full glass of water. The water evaporates as time passes. TIM: And that depends mainly on air temperature. Heat makes molecules move faster. So the warmer it is, the more water molecules shake loose and evaporate. That's the reason summer days tend to get more humid. An animation shows the pond scene and a thermometer. The temperature increases on the thermometer. More water molecules lift into the air from the pond. MOBY: Beep? TIM: Compared to other molecules that make up the air, water vapor is lightweight. So it rises through them. An animation shows a water molecule evaporating. It passes other molecules in the air. TIM: But the higher we go, the colder it gets, slowing the vapor molecules down. The temperature on the thermometer drops. The rising molecule slows down. TIM: They start to condense around microscopic particles. It can be a speck of dust or a single bacterium! Vapor molecules combine around a particle and a bacterium. TIM: Once that happens they're no longer vapor--they're tiny, invisible bits of liquid. When enough of them clump together, we can see them as clouds! An animation shows tiny specks of liquid in the air. The bits of liquid form a cloud. TIM: If the droplets in a cloud grow heavy enough, they fall to the ground. That's called precipitation -- also known as rain, or snow if it's cold. Precipitation is the last leg of the water cycle. Rain falls from the cloud. TIM: Then it repeats itself, over and over, again and again. The rain stops. The cycle of evaporation, cloud formation, and precipitation repeats. MOBY: Beep? TIM: We can measure humidity in a couple different ways. You've probably seen weather reports that show it as a percentage. That’s relative humidity. An animation shows Tim's watch. It says the temperature is 26 degrees Fahrenheit and the humidity is 75 percent. TIM: It compares the current amount of vapor to the maximum possible amount. That is, how much there would be if the system reached equilibrium. An animation shows a fraction with three vapor molecules in the numerator and four vapor molecules in the denominator. TIM: So, let's say there's this much vapor in the air now. The three vapor molecules in the numerator light up. TIM: If conditions stayed constant, it'd eventually balance out at this much. The four vapor molecules in the denominator light up. TIM: Divide the current amount of vapor by this potential amount, and you've got relative humidity. An animation shows the equation three divided by four equals point seven five. Point seven five converts to seventy-five percent. TIM: Only problem is, you have to recalculate every time the temperature changes. An animation shows the fraction three over four molecules, a thermometer, and the equation "relative humidity equals seventy-five percent." TIM: As it rises, more energy is available for water molecules to break loose as vapor. The temperature rises. Two more molecules appear in the fraction's denominator. It's now three molecules over six molecules. The relative humidity changes to fifty percent. TIM: So the potential amount of vapor increases. The animation highlights the six molecules in the denominator. It's the part of the fraction that represents the potential amount of vapor. TIM: A few degrees higher, and it increases even more. The temperature increases. Eleven more molecules appear in the denominator of the fraction. TIM: The current humidity increases, too, but usually not as quickly. Four more molecules appear in the numerator of the fraction. The relative humidity changes to thirty-five percent. TIM: The warmer it gets, the bigger the gap between current and maximum humidity. So while the amount of vapor is increasing, relative humidity goes down. Fifteen more molecules appear in the denominator. Only three more molecules appear in the numerator. The relative humidity becomes twenty-nine percent. MOBY: Beep! TIM: I know, it's kind of like, the opposite of how you'd expect it to work. That's one reason meteorologists—weather scientists—don't really like to use relative humidity. A meteorologist breaks through the relative humidity calculations on the screen. He reveals an image of the pond scene and thermometer. TIM: Instead, they look at current conditions and ask one simple question: More evaporation than condensation happens in the pond scene. The temperature is about 73 degrees. TIM: How cold would it have to get for condensation to equal evaporation? In other words, at what temperature would they reach equilibrium? The temperature drops to 54 degrees. Evaporation and condensation balance out. TIM: We call this the dew point. Any cooler, and you get more water going down than up. The temperature drops to about 48 degrees. More condensation occurs than evaporation. TIM: Early in the morning, before dawn, temperatures often dip below the dew point. That's why dew forms on grass and other surfaces. An animation shows a neighborhood in the morning before sunup. The plants have dew on their leaves. MOBY: Beep? TIM: Or frost, if it's cold enough. An animation shows Moby scraping frost off the window of their car. TIM: Unlike relative humidity, dew point describes the actual amount of vapor in the air. A dew point of 70 degrees means there's this much vapor… An animation shows Moby in an outdoor scene. The thermometer measures 70 degrees. Moby gestures to the beaker he is holding. It is filled with water. TIM: …in this much air. A cube outlines a portion of the air. It is filled with vapor molecules. TIM: That'll feel really muggy. While a dew point of 50 means there's about half as much vapor. So it feels nice and dry. The thermometer drops to 50 degrees. The water level in Moby's beaker decreases by half. The cube of air has half as many vapor molecules as before. MOBY: Beep? Moby points to the introduction letter. TIM: Right, the reason humid heat feels uncomfortable. On hot days, we cool off by sweating. An animations shows a skateboarder sweating on a hot day. TIM: The sweat absorbs body heat, which speeds up its water molecules. When they evaporate they take some heat with them, so we feel cooler. An animation shows the sweat droplets on the skateboarder's arm. The molecules in the sweat droplet speed up and evaporate. TIM: But when it's really hot and humid outside, evaporation is slower. If the temperature is close to the dew point, sweat won't go anywhere. We’ll be covered in warm, sticky liquid… yuck! An animation shows the same skateboarder on a hotter day. The molecules in the sweat move more slowly. Only a few molecules evaporate. TIM: What's this now? Moby hands Tim another letter. TIM: Dear Tim and Moby, For your information, Canada offers a wealth of winter vacation opportunities. Please cease and desist your defamation of our great land's recreational potential, or suffer the legal consequences. Sorry, The Canadian Office of Tourism TIM: Huh. Tim and Moby continue their journey to the Yukon. Category:BrainPOP Transcripts Category:BrainPOP Science Transcripts